TEMPORAL ALTERATION OF DOSE-DEPENDENT RESPONSE TO ACTIVIN IN NEWT ANIMAL-CAP EXPLANTS

In this study, we examined the dose-dependent responses of animal-pole cells of the Japanese newt, Cynops pyrrhogaster, to activin. Cynops h as a slower developmental rate and a simpler animal cap structure than Xenopus. These features enable temporal differences in animal-cap com petence to be identified more easily and relatively sharp dose-respons e profiles can be obtained without cell dissociation. When Cynops caps were excised at the mid-blastula stage and transcript levels of marke r genes were examined at the early gastrula stage, the strongest induc tion of brachyury occurred at a low activin dose, suggesting that cell s can recognize changing concentrations of an inducing signal in the e mbryo. Furthermore, the time course of brachyury expression revealed t hat caps from the mid-blastula stage exposed to a high dose of activin maintained a low expression level after induction. This suggests that Cynops animal-pole cells can assess activin concentration In a simple and direct manner. In addition, we found that animal-cap competence s ignificantly changes during the blastula stage. The data presented her e suggest that this change does not autonomously occur within animal-p ole cells but requires signals that emanate from other germ layers.